Photographic objectives



y 1961 J. G. BAKER 2,986,071

morocmpmc OBJECTIVES Filed Feb. 10, 1956 :s Sheets-Sheet 2 STOP 15 R 1 2L l2 2 Lens R0d'11 Thicknesses n v Gloss Types I R 0.631 1, 0.0791.70065 47.9 701479 [1' R -I.I26 f 0.007 1.64900 33.9 649339 V R -o.366'1 0.015 I.5l868 64.2 519642 R" -O.43| S 0.002

The stop lies 0.101 from the vertex of R toward the vertex of R S =ihebuck focal distance.

D= Length of IO waves of sodium light.

2 l3 R; D D

'3 12 [1111 TOR.

Mil BY y 1961 J. G. BAKER 2,986,071

PHOTOGRAPHIC OBJECTIVES Filed Feb. 10, 1956 3 Sheets-Shoot 3 Lens RadiiThicknesses n v Glass Types 1X I R 0.535 t, 0.075 |.755t0 47.2 755472 Rl.667 t 0.007 1.68900 30.9 689309 R L06! 5, 0.002

[U R 0.3|2 t 0.050 |.755IO 47.2 755472 I] R 0.8l7 t 0.0l3 LSOSOO 37.9605379 Y R -0.39s 0.013 |.6203l 50.3 520503 R |.e09 S 0.020

II. R o.s0s t 0.054 1.50370 4|.a 8044i 0 111 R, 5.00| t 0.007 |.7200029.3 720293 E11 R 0.435 t 0.075 1.74450 45.5 745455 K R piano t 0.020|.5I700 64.5 5|7s45 R piano 3 0.050

The stop lies 0.||47 from the vertex of R toward the vertex of R =theback focal distance.

The surface R ls aspheric and is of such 0 shape that at 0.32 radiansoff-axis, the thickness reaches a maximum; at 0.42 radians off-axis, thethickness is approximately equal to the thickness at the optical axis.The maximum variation in thickness is approximately 0.0!! of the focallength.

VENI'OR.

FIG. 3. a... i 5%. 84m gal/36; 6M4:

United States Patent PHOTOGRAPHIC OBJECTIVES James G. Baker, WestSomerville, Mass, assignor to The Perkin-Elmer Corporation, Norwalk,Conn., a corporation of New York Filed Feb. 10, 1956, Ser. No. 564,704

Claims. (CI. 88-57) This invention relates to photographic objectives ofthe Gauss type, which comprise a pair of meniscus components of netdivergent effect lying between collective components and concave to eachother on opposite sides of a central stop. More particularly, theinvention is concerned with a novel objective of the kind stated, whichis characterized by unusual excellence in its correction both for thelower order aberrations and for oblique spherical aberration and higherorder astigmatism. The new objective is especially adapted to meet therequirements of modern night aerial photography for an objective ofmedium focal length, speed, and coverage as typified by a 12" f/2.5 lenscovering a 55 degree total field, and examples of the objective in thatform will, accordingly, be illustrated and described in detail forpurposes of explanation.

It has long been known that Gauss objectives are capable of providing alarge well-corrected field for both visual and photographic purposes andthat various modifications of such objectives permit correctionaccording to the uses to which the objectives are to be put. However,these objectives have deficiencies limiting their speed and coverage anda notable example of such a deficiency is the appearance of sphericalaberration at considerable off-axis angles, which is referred to asoblique spherical aberration and is merely the reappearance of theon-axis spherical aberration amenable to control. Oblique sphericalaberration generally varies as the square of the field angle off-axisand as the cube of the relative aperture, so that, in a Gauss lens,which covers a large angular field at high speed, serious difficultiesarising from oblique spherical aberration can be anticipated and suchdifficulties are intensified, if the focal lengths required areappreciably greater than those generally used for ordinary photography.

Another deficiency in Gauss objectives is the tendency of the higherorder astigmatism to deteriorate rapidly with an increase in fieldangle. This tendency is so pronounced that unusual corrective means forcontrol of the aberration must be utilized in lenses for the purposesfor which the new objectives are employed.

In my Patent 2,532,751, issued December 5, 1950, I have shown how anincrease in the central air space of a Gauss objective can be employedto reduce oblique spherical aberration by eliminating most of that partof the aberration varying as the cube of the aperture and limiting itscontributions to residuals varying as the fifth and higher orders of theaperture. Also, in Patent 2,671,380, issued March 9, 1954, I have shownhow compensating "ice means may be introduced into the central air spaceof such an objective to achieve advantages including the correction ofoblique spherical aberration. Thus, by use of the expedients disclosedin the patents, oblique spherical aberration can be reduced in amplitudeand its contribution for any given zone and field angle may be broughtapproximately to zero.

In work on the development of a Gauss objective in the form of a 12"f/2.5 night lens covering a 55 degree field, I have found that such alens constructed in accordance with the teachings of the patents fallsshort of optimum performance. In fact, it appears that the use of therelatively strongly-curved surfaces around the central stop, which arerequired in such objectives having a large central air space, leads toan actual increase in the oblique spherical aberration in the skewdirection. The normal oblique spherical aberration of such an objectivehaving flattened curves around the central stop causes a considerableextension in the tangential direction in the image of a point-source foronly a moderate extension in the skew direction, while such objectiveshaving a large central air space and the required steeper curves haveless oblique spherical aberration in the tangential direction and morein the skew. In the latter objectives, the rate of change in theaberration over the field is comparatively small, so that the objectivesyield off-axis images of excellent quality. However, if a still furtherimprovement in off-axis image quality is to be obtained, the obliquespherical aberration in the skew direction must be further reduced,while the correction of that aberration in the tangential direction ismaintained.

The present invention is directed to the provision of a novel objectiveof the Gauss type, which is superior in performance to the objectives ofthe patents and in which a number of expedients have been employed in anew combination to achieve the desired result. The objective comprises apair of outer components of net collective effect, which may be simpleelements or compound components, and a pair of inner meniscus componentsof net dispersive effect lying concave to each other and each composedof two or more elements. The surfaces around the central stop are ofrelatively weak curvature and materials of medium to high index ofrefraction are employed for the positive elements of both componentswith the result that the lens speed is maintained despite the shallowcurves. in addition, the dioptric powers of the respective outercollective components and of the concave surfaces adjacent the centralstop are all kept within specified limits and, when the outer collectivecomponents are cemented, the index differences across the cementedsurfaces are restricted. Various other expedients employed in the newobjective to insure its extraordinarily excellent performance will bepointed out in the detailed description to follow.

For a better understanding of the invention, reference may be made tothe accompanying drawings, in which Figs. 1 and 2 are diagrammatic viewsof two different lenses embodying the invention with tables of data forthe lenses;

Figs. 1A and 2A are fragmentary views on an enlarged scale showing theuse of aspheric curvature on two of the1 surfaces of the lenses of Figs.1 and 2, respectively; an

Fig. 3 is a diagrammatic view of a modification of the lens of Fig. l,with which a field flattener is used, and a table of data for themodified system.

The form of the new objective illustrated in Fig. 1 has been found to beoptimum for a 12" f/2.5 night lcns covering a 55 degree field and it isof the Gauss construction. The lens includes outer collective componentsin the form of cemented doublets, of which the one in front or on thelong conjugate side is made up of an outer collective element I and aninner dispersive element 11, while the doublet at the rear or on theshort conjugate side is made up of an inner dispersive element VII andan outer collective element Vlll. The collective components surroundnegative meniscus components, which are concave toward each other aroundthe central stop and are each made of a pair of separated meniscuselements instead of cemented doublets, as is common in prior Gausslenses. The front meniscus component consists of an outer positiveelement III and an inner dispersive element IV, while the rear meniscuscomponent consists of an inner dispersive element V and an outercollective element VI. In the lenses illustrated, the corrective actionfor oblique spherical aberration has been obtained by the use ofcemented doublets as the outer collective components, although simpleelements with aspheric surfaces could also be employed for the purpose.The positive elements I, III, VI, and VIII of the lens are made of rareearth glasses, although the principles of the invention may be employedin lenses, in which materials of lower index of refraction are used forthe positive elements. The improvement gained by the use of the rareearth glasses lies primarily in the added perfection of correction for agiven speed, focal length, and coverage.

The lens system of Fig. 2 is closely similar to that of Fig. l withrespect to the nature of the components and the elements thereof, butdiffers from the Fig. 1 system in that the glasses employed are ofmedium to high index and rare earth glasses are not employed. Thecorrections of the Fig. 2 lens are different from those of the lens ofFig. l and are not markedly inferior. However. in modern photographicobjectives, in which a goal of absolute perfection is sought, a gain ofeven 20% in performance achieved by the use of the rare earth glasses ofthe Fig. 1 construction may justify use of such materials.

The invention is based on the use of materials of medium to high indexfor the collective components and the objectives of Figs. 1 and 2 aretypical emodiments of the invention, in which the materials for thosecomponents lie within the upper part of the practical index range. If apartial sacrifice of lens speed is permissible. objectives embodying theinvention and utilizing the older types of optical glass of medium tolow index may be constructed to give a performance otherwise comparableto those of the lenses of Figs. 1 and 2.

In Gauss objectives, the tangential pencils as a rule suffer greateraberrations and a wider range of aberrations over the field that theskew pencils, since the angles of refraction reach maximum value in themeridional plane at practically every surface. Also. the more any givenradius of curvature departs from being concentric around the image ofthe stop in its own medium, the wider the difference between thetangential and skew refractions, which difference is dependent upon themagnitude of the angles of incidence and refraction of the chief rays ofthe pencils. On the other hand, the corrections for the tangential raysare more responsive than the skew corrections to compensatingrefractions. Accordingly, I have found it possible by employing a numberof elements with comparatively shallow curves with compensatingrefractions properly arranged in curvature. location, and indexdifference to effect the necessary corrections for the oblique sphericalaberration of the tan- Ill gential rays. The use of such shallow curvesthen provides the minimum oblique spherical aberration in the skewdirection. Additional corrections of considerable value might beobtained by the used of aspheric surfaces, but, in lenses to bemanufactured in quantity, such extensive departures from sphericalsurfaces are not presently feasible.

The objectives of Figs. 1 and 2 have relatively shallow curves comparedto prior objectives, and, while shallower curves than those used in thetwo objectives could be adopted, the use of such shallower curves mightresult in a system of excessive overall length. Materials of high indexordinarily have a greater absorption of light than low index glassesand, if such high index materials are to be used, economy of lightrequires that the bulk of the optical system be minimized. The systemsof Figs. 1 and 2 are, accordingly, relatively compact for the purpose ofkeeping the total length of the path through glass to a minimum incompromise with the state of correction. Another advantage of a compactoptical system is that it generally involves less vignetting at greatoffaxis angles, since the various bundles of light generally lie withinsmall solid angles with relation to the entrance pupil. A compact lensis moreover preferable for general use.

In constructing objectives in accordance with the invention, a number ofvariations within ranges as follows may be adopted. In lenses, such asthose of Figs. 1 and 2, in which the outer collective components arecemented doublets, the index differences across the first and lastcemented surfaces may vary considerably. In the first componentconsisting of the elements I, II, element I should have the higher indexand the index dilference across the cemented surface defined by theradius R; should be fairly substantial and lie within the range between0.03 and 0.08. An index difference less than 0.03 may cause excessivecurvature of the surface with resultant excessive thickness of thecomponent or overcorrection by the surface, whereas too great an indexdifference causes both distortion and astigmatism to appear. In the caseof the outer collective component at the rear of the system, theconverging pencils of light require a greater curvature of the cementedsurface in order to obtain the desired corrective action and, because ofthe greater curvature for high inclination, less index difference isdesirable. In the rear collective component, the element VII should havethe lower index and the preferred range for the index difference acrossthe surface defined by the radius R has been found to lie between 0.01and 0.06. It should be noted that, in outer collective components moreelaborate than cemented doublets, the index differences mentioned applyto the cemented surfaces curved away from the stop, since the curvaturesof other surfaces curved around the central stop are insensitive and ofno great corrective action for the purposes of this invention.

As indicated above, the average index for the outer collectivecomponents should be as high as practicable, although excellentobjectives embodying the invention can be Obtained provided the averageindex is at least 1.58. As materials having an index in excess of 1.80are at present notably yellow and, therefore, undesirable, a range of1.58 to 1.80 may be assigned for the average index of both the front andrear collective components. The term average index as here used isintended to refer to the arithmetical mean of the indices of all theelements of a component, so that, if the component is compound and withor without aspheric surfaces, its average index is the arithmetical meanof the indices of the elements. If the component is a single element,the index of the element is regarded as its average index. while. if thecomponent is a cemented triplet, for example. the average index is thearithmetical mean of the three indices,

The use in the new objective of strong doublets for the outer collectivecomponents permits easy color correction and thus imposes lessrestriction on the choice of materials for the negative meniscuselements adjacent to the central stop. Such negative meniscus elementscan, accordingly, have a lower index than is usual in Gauss objectiveswith a resultant advantage to the Petzval sum. The use of lower indexmaterials for the negative meniscus elements would ordinarily requireincreased curvatures of the air surfaces adjacent to the central stopand this in turn would have a detrimental effect on the obliquespherical aberration and the on-axis spherical zone. However, in theobjectives of the invention, there is a broken contact between theelements of the negative meniscus components and this permits retentionof moderate curvatures on the surfaces adjacent the central stop. I havefound that the front negative meniscus component should have at leastone negative element with an index, which is less than the average indexof the front collective component by 0.02 to 0.15. Similarly, the rearnegative meniscus component should have at least one negative elementwith an index, which is less than the average index of the rearcollective component by 0.05 to 0.20.

Another feature of objectives of the invention is that the indices ofthe positive elements of the negative meniscus components can be highwithout introducing excessive astigmatism, since the elements are ofmeniscus form. For best results, the highest index of any of thepositive elements within the negative meniscus components should begreater than the average index of either the front or the rearcollective component and the highest index of the positive elementswithin either negative meniscus component should exceed the lowest indexof the negative elements within that component by from 0.03 to 0.25.This favorable distribution of indices must be kept within bounds inorder to avoid an unbalance within the system and an upper limit of 1.85may be placed upon the indices of the positive elements of the negativemeniscus components. In this connection, it may be noted that element VIof the lens of Fig. l is made of a glass of index 1.8.

The limitation of certain constructional features other than the indicesof the materials is necessary to define the objectives of the inventionand one such feature, which must be confined within definite limits, isthe effective lens power ascribed to the collective components where thelens thickness is ignored. In the new objectives, the range of power ofthe thin-lens equivalent of the front collective component is from 0.45to 0.75 in terms of the power of the entire system taken as unity.Similarly, the range of power for the rear collective component is 0.75to 1.10 on the same basis, the range being determined primarily byrequirements for correction of distortion and coma,

Another useful limitation defining the structure of the collectivecomponents is based on the general degree of bending of the individualcomponents. In the front collective component, a definite meniscustendency is required and such tendency can best be insured by limitingthe dioptric power of the innermost air glass refracting surface, thatis, the surface defined by the radius R to the range from 0.2 to 0.8,the power being defined in terms of the power of the entire system takenas unity. The rear collective component can be only weakly meniscus andmore generally is weakly biconvex. These characteristics of thecomponent may be defined by specifying that the limit of power of theinner air glass surface lies in the range from 0.3 to +0.4 of the powerof the entire system taken as unit. In referring to the power of anindividual lens surface, I intend that the term is to be considered inits usual dioptric sense.

When the lens powers and bendings for the collective components havebeen restricted, as above set forth, the limitations on the form of thenegative meniscus components involve chiefly bending. With the Petzvalsum reasonably well corrected as in high quality objectives,

the powers of the negative meniscus components are essentially defined,so that consideration of the bendings only remains. Such bendings canbest be described by reference to the dioptric powers of the concavesurfaces around the central stop and, because of the convergence ofbundles of pencils from the long conjugate side, the negative power ofthe front concave surface is appreciably greater than the power of therear surface. I have found that, for a favorable flatness of field andcorrection of spherical aberration, the lens power of the front concavesurface adjacent the central stop, that is, the surface defined by theradius R should lie within the range from l.7 to -2.8. The preferredlens power for the rear concave surface adjacent to the central stop,that is, the surface defined by radius R lies between -l.0 and 1.90. Inaddition, a favorable correction for coma can be readily obtained, whenthe absolute power of the front concave surface in objectives of theinvention is greater than that of the rear concave surface.

A further feature of objectives of the invention, which must be keptwithin specified limits, is the length of the central air spacecontaining the stop. I have found that, for favorable correction of theoblique spherical aberration in the skew direction, an air space withinthe range of 0.14 F to 0.28 F is to be preferred, F being the focallength of the system. If a shorter air space is employed, the tangentialoblique spherical aberration tends to become uncontrolled, while, if theair space is greater than the top limit stated, a considerable loss ofoverall lens power results and this demands a general steepening of thecurves with consequent aberrations.

In my work in the development of the objectives of the invention, I haveobserved that quite often the upper and lower rim ray corrections can bematerially improved by judicious use of aspheric corrections on theappropriate surfaces. In general, when the lens barrel has an overalllength which is a substantial fraction of the focal length of thesystem, the inclined bundles of rays passing through the entrance pupilstrike the front and rear collective components quite far from theoptical axis. The extreme rays use the outermost portions of suchsurfaces in areas not used at all by the bundles toward the central partof the field. Accordingly, it is possible to make use of asphericcorrections in the peripheral zones of such surfaces to influence imageformation far off axis without affecting the performance in the centralportions of the field. Frequently, it is adequate to select only onesuch surface in the front portion and another such surface in the rearportion of the lens system. The use of aspheric corrections may beregarded as the addition of a very weak lens element superimposed on aselected lens surface and such an element may be of either positive ornegative effect on the particular ray, as required in common with theother design properties of the system. The effectiveness of the asphericcorrection utilized in this manner arises from its nearly completeindependence of the other parameters already heavily burdened incontrolling overall performance. If an objective of the invention isproperly designed, the aspheric corrections mentioned can be kept inreserve to be used with the other factors set forth for the purpose ofobtaining excellent off-axis images in the extreme corners of theformat. In practice, it is far more desirable to employ turned-downedges than turned-up edges, since the former are more easily made, butedges of either sort may be required.

In systems of the invention, the depth of the aspheric corrections atthe extreme margin of the clear aperture varies with the specific systemand may be only part of the length of a wave of sodium light atwavelength 5893 angstroms. In a 12" f/2.5 lens covering a 55 degreefield, the aspheric corrections should have a depth equal to the lengthof at least five such waves. By depth," I mean the distance at themargin of the aperture in a direction parallel to the optical axisbetween the basic spherical surface and the superimposed asphericsurface. While a definite upper limit to the aspheric variation cannotbe precisely stated, it would be unusual if aspheric corectionsamounting in axial depth at the margin of the clear aperture to a lengthof more than 200 waves of sodium light were necessary to achieve a finaloff-axis correction and, if the maximum axial depth or sagitta of theaspheric correction relative to the basic sphere were greater than thelength of 200 waves of sodium light, it is unlikely that the performanceof the objective in the intermediate field would be satisfactory.

In Fig. I, the dotted lines L on the element II indicate a turned-downedge (greatly exaggerated) on the surface defined by the radius R whichmakes that surface aspheric. The dotted lines L; on element VIIsimilarly indicate a turned-down edged (greatly exaggerated) on thesurface defined by radius R The turning down of the edges providesaspheric corrections at the margins of the clear aperture and, in thelens of Fig. 1, such corrections have an axial depth D equal to thelength of approximately ten waves of sodium light. In the lens of Fig.2, the dotted lines L and L indicate turned-down edges on the surfacesdefined by the radii R and R12. respectively, and the turning down ofthe edges provide aspheric corrections having an axial depth D equal tothe length of approximately ten waves of sodium light.

The constructional data of the objective of Fig. l are substantially aslisted in the following tabulations wherein:

Roman numerals indicate elements of lens assemblies;

The symbol R indicates the radius of curvature for the optical surfaces;

1 indicates the axial thickness of optical elements;

S indicates axial separations between adjacent elements;

71 indicates the index of refraction of the glass at the sodium line;

v indicates the reciprocal dispersion of the optical elements; and

The Glass Types" are conventional international code numbers, the firstthree numerals indicating the index of refraction (less 1) and the nextthree numerals indicating the reciprocal dispersion (without the decimalpoint).

The stop lies 0.1147 from the vertex of R toward the Vertex of R S =theback focal distance.

The constructional data of the objective of Figs. 2 are substantially asfollows:

Example II Lens Radii Thlckn, 17 Glass nesses Types s.=0.002 R: 0.331 mt; =0.053 1.70065 47.8 701470 S1==0.029 R0 0.960 W 1. =0.015 1.0400033.0 040338 s,=o. 171 R, =-0.30s 1, =0.015 1.51003 04.2 519642 s,=0.017Rw=-0.727 v1 t. =0.0s1 1.70005 47.0 701470 s.=0.002 Rn: 2.170 v11 t=0.00s 1.68900 30.0 080309 R"=1.0ss

The stop hes 0.101 from the Vertex of R toward the Vertex of R S =theback focal distance.

After all the various improvements above described have been combinedwith satisfactory forms of correction for the standard aberrations,there remains a zonal term in the field curvature that is irreducible byany means within the objective so far determined. In the absence ofastigmatism, the zonal departure from a flat focal plane for a fullfield of one full radian amounts in amplitude to about 0.0036 of thefocal length and reaches this maximum value at about 0.70 of thedistance from the axis to the edge of the specified field. Wherenecessary, the zonal field curvature referred to can be eliminated bythe use of a field flattening lens.

It is well known that an auxiliary field flattener can be placed nearthe focal plane of an objective to aid in eliminating the zonal fieldcurvature without at the same time affecting the optical correctionssignificantly, particularly if the latter are balanced with the fieldflattener in place. Such a field flattening lens can also be used toreduce the Petzval sum of the third order of approximation and this thenrequires that some weak negative dioptric power be given to the lens. Anaspheric deformation can be added to the field flattener to complete thetask of flattening the field.

The lens illustrated in Fig. 3 is that shown in Fig. l employed with anauxiliary field-flattener for the purpose of eliminating the residualzonal field curvature. The example of Fig. 3 is typical, but, withoutdeparting from the spirit of the invention, the distance of the fieldflattening lens from the focal surface can be varied or the sphericaland aspheric portions of the lens surface can be distributed wholly orin part between the front and the back of the lens. Also, it is possibleto utilize a compound lens with spherical or aspheric surfaces in sucha' position near the focal plane as to achieve improved correction forfield-flatness and elimination of chromatic distortion and residuallateral color.

In general, the field flattener must be quite close to the focal planein order that the field flattening action may not affect otheraberrations irreparably. Occasionally, it may be desirable to have thelast surface of the field flattener adjacent the focal plane actually incontact with the plane and, when the back side of the field flattener isflat and coincident with the focal plane, it may serve as a referenceplane against which a photographic emulsion or a reticle may be placed.More often, the field flattener will be purposely spaced a shortdistance from the focal plane to keep dust and defects of polish fromappearing on the optical image or to allow space for the use of a focalplane shutter, a calibration plate, a filter, etc. If the rear surfaceof the field flattener lies a distance greater than 0.15 F from thefocal plane, it loses its corrective powers for field flatteningpurposes and becomes essentially an element of the optical system ratherthan a field flattener. Accordingly, the location of the field flattenermay be defined by specifying that its rear surface lies on the longconjugate side of the focal plane by a distance varying from F to 0.15F.

The field flattener may be a very weak spherical or aspheric lens usedin the design of the main system as a final correction on a performancefound satisfactory except for residual field curvature, or the fieldflattener may have an appreciable negative lens power for the purpose ofhelping to correct the basic Petzval curvature of field. If a fieldflattening lens has a variation in thickness at any point which exceedsa value equal, for example, to 0.04 F, the lens loses its essentialcharacter as a field flattener and becomes a strong element of the lenssystem. In general, if the field flattener is close to the focal planeand is made of a material of an average index of refraction, the maximumvariation in focal displacement caused by variation in the thickness ofthe flattener is of the order of 0.015 F. For a focal length equal to100 mm., a field flattener limited in variation of thickness to amaximum of 0.04 F produces a corrective power on field curvature of theorder of 1.5 mm., which is in the normal range of the residual fieldcurvature of optical systems for general use. Therefore, in order thatthe effect of the field flattening lens employed with objectives of theinvention may be confined to field flattening, the variation inthickness of the lens must lie within the range from approximately 0 to0.04 F. The lower limit of approximately 0 is used, because occasionallythe aspheric correction on the field flattener may have an axial depthamounting to only a few waves of sodium light and, in the limit, thethickness of the field flattener may be used to efiect a finalcorrection of small magnitude on distortion and astigmatism in the outerfield.

Although, as shown above, the upper limit of the variation in thicknessof the field flattener has been found to be 0.04 F, not all of thisvariation can properly be employed in the aspheric portion. The asphericcorrection on the field flattener is employed for removing fieldcurvature in the higher order terms, whereas any basic lens power of theflattener is immediately useful for reducing the Petzval sum with anadded effect on the higher order terms. Accordingly, an upper limit of0.02 F can be placed upon the aspheric variation of the field flattener,such limit referring to the maximum axial distance between the asphericsurface and a spherical surface with its center on the optical axis andpassing through the vertex and the extreme extension of the asphericsurface.

substantially as follows:

Example III Lens Radil Thickn. 0 Glass nesses Types R I 0.635 I 1 =0.075 1. 75510 47. 2 755472 R: 1.667 11 is =0. 007 1. 68900 30. 9 089309Sr=0. 002 R 0.312 In i; =0. 050 I. 75510 47. 2 755672 S:=0. 020 R5 0.817IV t =0. 013 1. 60500 37. 9 605379 S;=0. 200 Bl 0.399 v is =0. 013 1.6203i 60. 3 620603 S =0. 020 R 0.909 VI ls =0. 054 1. 80370 41. 8 804418S5=0. 002 Ru= 5.001 VII t =0. 007 l. 72000 29. 3 720293 Rrg= 0.435 VIIIis =0. 075 l. 74 #50 i5. 8 745458 Se=0. 610 R15= 1318110 Ix [9 =0. 0201.5l700 64. 5 H7645 R plnno The stop lies 0.1147 from vertex of R S =theback focal distance.

The surface of R is aspheric and is of such a shape that at 0.32 radianofi-axis, the thickness reaches a maximum; at 0.42 radian ofi-axis, thethickness is approximately equal to the thickness at the optical axis.The maximum variation in thickness is approximately 0.011 of the focallength.

Then lens in Example I may be used, where necessary, without a fieldflattener, but a field flattener is employed if ideal results are to beobtained. In order that the lens may be used without the fieldflattener, it is so constructed that the astigmatic surfaces coincideover most of the field leaving only the zonal field curvature to becorrected.

I claim:

1. An objective for photographic purposes corrected for spherical andchromatic aberrations, including oblique spherical aberration, coma,astigmatism, field curvature, and distortion, which comprises a pair ofouter components of net collective effect and a pair of components ofnet negative effect and of meniscus form disposed between the outercomponents and with their concave surfaces opposed to each other onopposite sides of a the vertex of R toward the central sto bothcollective components having an average index 0 refraction from 1.58 to1.80 and the front collective component being of miniscus form with itsinner air surface concave and of a dioptric power from 0.2 to -O.8, thesaid front collective component having at least one cemented surfacecurved away from the stop with the largest index difference across sucha surface lying within the range 0.03 to 0.08 and being caused by adecrease in index in the direction of light travel while the rearcollective component has a front air surface of a dioptric power from0.3 to 0.4. said rear collective component having at least one cementedsurface curved away from the stop, the negative meniscus components eachincluding at least one positive and one negative element with thehighest index of refraction of each positive element in each meniscuscomponent exceeding the lowest index of refraction of each negativeelement of that component by at least 0.03 but less than 0.25, the

front negative meniscus component having a concave surface adjacent thestop of a dioptric power from l.7 to 2.8 and the dioptric power of aconcave surface adjacent the stop of the rear negative meniscuscomponent varying from L to 1.9 and having a numerical value less thanthat of said concave surface of the front negative meniscus component,all said dioptric powers being stated in terms of the net power of theentire objective taken as unity, the central air space separating saidconcave surfaces of the negative meniscus components having a lengthgreater than 0.14 F and less than 0.28 F, F being the focal length ofthe objective.

2. The objective of claim 1, in which the largest index differencesacross the said curved cemented surface of the rear collective componentlies within the range 0.01 to 0.06 and being caused by an increase inindex in the direction of light travel.

3. The objective of claim 1, in which the front negative meniscuscomponent has at least one negative element of an index of refractionsmaller than the average index of the front collective component by aquantity lying within the range 0.02 to 0.15.

4. The objective of claim 1, in which the rear negative meniscuscomponent has at least one negative element of an index of refractionsmaller than the average index of the rear collective component by aquantity lying within the range 0.05 to 0.20.

5. The objective of claim 1, in which the largest index of refraction ofthe positive elements within the negative meniscus components is greaterthan the average index of each collective component but less than 1.85.

6. The objective of claim 1, in which the dioptric power of the frontcollective component as computed from the curvatures with thethicknesses neglected lies within the range 0.45 to 0.75 and thedioptric power of the rear collective component similarly computed lieswithin the range 0.75 to 1.10.

7. An objective for photographic purposes corrected for spherical andchromatic aberrations, including oblique spherical aberration, coma,astigmatism, field curvature, and distortion, which comprises a pair ofouter components of net collective effect and a pair of components ofnet negative effect and of meniscus form disposed between the outercomponents and with their concave surfaces opposed to each other onopposite sides of a cent r a1 stop, both collective components having anaverage index of refraction from 1.58 to 1.80 and the front collectivecomponent being of meniscus form with its inner air surface concave andof a dioptric power from 0.2 to 0.8, while the rear collective componenthas a front air surface of a dioptric power from 0.3 to 0.4, said rearcollective component having at least one cemented surface curved awayfrom the stop, the negative meniscus components each including at leastone positive and one negative element with the highest index ofrefraction of each positive element in each meniscus component exceedingthe lowest index of refraction of each negative element of thatcomponent by at least 0.03 but less than 0.25, the front negativemeniscus component having a concave surface adjacent the stop of adioptric power from l .7 to 2.8 and the dioptric power of the concavesurface adjacent the stop of the rear negative meniscus componentvarying from l.0 to l.9 and having a numerical value less than that ofsaid concave surface of the front negative meniscus component, all saiddioptric powers being stated in terms of the net power of the entireobjective taken as unity, the central air space separating said concavesurfaces of the negative meniscus components having a length greaterthan 0.14 F and less thin 0.28 F, F being the focal length of theobjective, the outer collective components and the negative meniscuscomponents between them forming a contiguous group and a fieldflattening lens lying outside the group and adjacent to and precedingthe focal surface with the vertex of the rear surface of the lens spacedalong the 12 optical axis from the focal surface by a distance rangingfrom 0 F to 0.08 F, the field flattening lens being aspheric on at leastone of its air surfaces by an amount not exceeding 0.02 F in depth ofdeparture of the aspheric surface from an imaginary spherical surfacehaving its center on the optical axis and passing through the vertex andthe extreme extension of the aspherie surface.

8. An ob ective having constructional data substantially as follows:

Lens Radll Thlckup I Glass masses Type:

R: II 1-667 11 :1 -0.007 1. 08900 30.0 089300 8 -0002 Rt" 0.312 111 at-0.050 1. 75510 47.2 750472 81-0020 Rs- 0.817 1v t1 0. 013 1 00500 37.0305370 s=-0.200 R; --0.390 v 21-0013 1.02031 00.3 020003 B0 II -l.609

51-0020 Raw-0.000 vi n-0.054 1.80370 41.3 some Ss=0.002 Rn- 5.001 VII11-11007 1.72000 20.3 720203 Rn- 0.43s VIII t. -0.075 1. 74450 45.8745450 The stop lies 0.1147 from the vertex of R toward the*Ve'fFeYBf'Rij S =the back focal distance, R=radius of curvature for theoptical surfaces, l=axial thickness of optical elements, S=axialseparations between adjacent elements, n =indeX of refraction of theglass at the sodium line. v=reciprocal dispersion of the opticalelements.

9. An objective having constructional data substan tially as follows:

Lens Radll Thtcb no I Glass nesses Types 8,-0.002 R4- 0.331 III ti-0.053 1.70065 47.8 701478 S -0.029 Rs= 0.960 IV t; -0.015 164900 33.8649838 81 0.171 Re I 0.366 V ts -0.015 1.51868 64.2 519642 81-0-1117 R0='-0.727 Vl is -0.061 1.70065 47.8 701478 Ss 0.002 R 2.170 VII 21-00061.68900 30.9 089309 Ris- 0.439 VIII ti -0.099 1.70065 47.8 701478 Thestop lies 0.101 from the vertex of R toward the vertex of R S the backfocal distance,

R=radius of curvature for theoptical surfaces, r=axial thickness ofoptical elements,

S=axial separations between adjacent elements,

13 n =index of refraction of the glass at the sodium line, v=reciprocaldispersion of the optical elements.

10. An objective having constructional data substantially as follows:

[F-LDOO f/2.5]

Inns Radlt Thickm; a Glass nesses Types s -0.002 R4- 0.312 III t; -0.0501.75510 47.2 765472 5 -0020 Bn- 0.817 IV 34 -0.013 1.60500 37.9 6053704-0020 R-0.909 VI t; -0.054 1.80370 41.8 804418 Ss-0.002 R1!- 5.001 VII0.007 1.72000 29.3 720293 1211- 0.435 VIII 1| -0.075 1.74450 45.8 745458Ru -(LQH 80-0640 Ruplsno IX is -0.020 1 51700 64.5 517045 Rnplane The atlies 0.1147 from the vertex of R toward the vertex of R Sq=th6 backfocal distance,

The surface R is aspheric and is of such a shape that at 0.32 radianoff-axis, the thickness reaches a maximum; at 0.42 radian off-axis, thethickness is approximately equal to the thickness at the optical axis.The maximum variation in thickness is approximately 0.011 of the focallength,

R=radius of curvature for the optical surfaces,

t=axial thickness of optical elements,

S=axial separations between adjacent elements,

n =index of refraction of the glass at the sodium line,

v=reciprocal dispersion of the optical elements.

References Cited in the file of this patent UNITED STATES PATENTS1,779,257 Lee Oct. 21, 1930 2,100,290 Lee Nov. 23, 1937 2,100,291 LeeNov. 23, 1937 2,289,779 Herzberger July 14, 1942 2,550,685 Garutso May1, 1951 2,559,881 Kingslake et a1 July 10, 1951 2,600,207 Cook June 10,1952 2,622,478 Kleineberg et al Dec. 23, 1952 2,683,396 Klemt et a1 July13, 1954 FOREIGN PATENTS 427,008 Great Britain Apr. 12, 1935

